The invention relates to power trains, particularly for use in motor vehicles for transmission of torque between a prime mover (such as an internal combustion engine) and one or more wheels. More particularly, the invention relates to improvements in power trains of the type wherein the means for transmitting at least a portion of torque from the rotary output element of a prime mover to a rotary driven element (such as the output shaft of an infinitely variable transmission) comprises a torque monitoring device or sensor with a normally annular plenum chamber for oil or another suitable hydraulic fluid. Still more particularly, the invention relates to improvements in power trains wherein the torque monitoring device or torque sensor comprises at least two elements forming part of a valve and being movable relative to each other to determine that pressure which is indicative of the torque being transmittable by the torque sensor. The valve communicates with the at least one plenum chamber and the torque sensor is preferably combined or combinable with an infinitely variable transmission of the type wherein an endless flexible element is trained over a pair of adjustable pulleys or sheaves.
Torque monitoring devices or sensors of the above outlined character are disclosed, for example, in German patent No. 28 28 347 as well as in published German patent applications Nos. 40 36 683, 42 34 294, 42 01 692 and 35 38 884. The torque sensors which are disclosed in these prior Publications are designed to establish a clamping engagement between component parts of a torque transmitting apparatus in dependency upon the load or in dependency upon the transmitted torque. As a rule, such torque sensors are designed to ensure a frictional engagement between component parts which are urged against or toward each other by a force depending at least upon the transmitted loads or upon the transmitted torque, namely in such a way that the force acting to urge the component parts into frictional engagement with one another at least approximates the force which is necessary for the transmission of torque. The application of an excessive force for urging the component parts against each other results in excessive wear whereas the application of an insufficient force entails a slip and hence again excessive wear between the parts which are maintained in frictional engagement with one another.
A conventional torque monitoring device or sensor constitutes, or can be said to constitute, a valve whose operation is dependent at least upon the magnitude of transmitted torque. Those portions of the torque sensor which are located downstream of the plenum chamber are constructed and assembled to constitute the aforementioned valve and the plenum chamber receives pressurized hydraulic fluid from a suitable pump. The valve acts as a flow restrictor or throttle which seals the path for the flow of fluid from the plenum chamber so that the pressure of fluid in the plenum chamber increases in response to abrupt rises (peaks) of transmitted torque. This entails a corresponding rise of fluid pressure in the plenum chamber as well as in the cylinder chamber or chambers of one or more cylinder and piston units which are provided to adjust the inifinitely variable transmission including a pair of adjustable sheaves and an endless flexible element which is trained over and serves to transmit torque between the adjustable sheaves. A rise of pressure in the cylinder chamber or chambers brings about a corresponding increase of the clamping force between the endless flexible element and the sheave or sheaves of the transmission. In other words, the frictional engagement between the sheave or sheaves on the one hand and the flexible element on the other hand increases in response to an increase of the transmitted torque and/or in response to the development of an abrupt rise or surge of transmitted torque.
In order to adjust the flow restrictor, heretofore known torque sensors comprise discs which are provided with confronting cam faces or ramps bearing upon rolling elements which are or which can be disposed between them with a force generated by the source of pressurized hydraulic fluid. If the transmitted torque develops peaks, especially peaks which develop in the torque being transmitted from the prime mover, the discs are caused to move axially and away from each other whereby an axially movable portion reduces the effective cross-sectional area of the outlet for the flow of hydraulic fluid from the plenum chamber at a rate which is proportional to the magnitude of the peaks of transmitted torque. In addition, the discs which are provided with the aforesaid cam faces or ramps serve as a means for mechanically transmitting at least a portion of the driving torque to adjust (such as reduce) the effective cross-sectional area of the outlet from the plenum chamber as a function of the magnitude of transmitted torque and to thus properly adjust the frictional clamping force between the adjustable sheaves and the endless flexible element of the infinitely variable transmission. The flexible element can constitute a belt or a chain; at this time, it is often preferred to employ a chain. As a rule, the flow restrictor permits the flow of hydraulic fluid therethrough except under certain circumstances of use of a prior torque sensor, such as in response to the development of highly pronounced peaks of transmitted torque. It follows that the pump or another suitable source of pressurized fluid must be capable of supplying the pressure which ensures a sufficiently pronounced engagement of the ramps or cam faces on the cam discs of the torqe sensor to transmit the required torque and which is needed to ensure that the fluid leaving the plenum chamber is maintained at a requisite pressure; the latter constitutes a permanent loss in pumping capacity.
It is also known to construct a torque sensor for use in the power train of a motor vehicle in such a way that the sensor can furnish a torque- and/or load-dependent pressure as well as a pressure which is a function of the ratio of the infinitely variable transmission. This necessitates a reducion of the clamping force upon the rolling elements between the faces or ramps of the clutch discs or analogous valving elements of a previously known torque sensor as well as between the endless flexible element and the adjustable sheaves of the infinitely variable transmission, especially when the power train is set up to operate within the partial-load range. Such design of the torque sensor renders it possible to reduce to a minimum losses developing as a result of frictional clamping engagement between he flexible element and the sheaves of the transmission. Thus, and when the infinitely variable transmission is set to operate in such a way that the looped portion of the flexible element which is trained over the torque transmitting (driving) sheave is located at a minimum radial distance from the rotational axis of this sheave, i.e., when the ratio of the transmission is reduced to a minimum value, the pressure supplied by the torque sensor is or can be greater than under those circumstances when the aforementioned looped portion of the flexible element is located at maximum radial distance from the axis of the driving sheave; at such time, the ratio of the infinitely variable transmission is maintained at a maximum value. It is assumed that the magnitude of the torque under the just outlined circumstances (namely when a looped portion of the flexible element is located at a minimal or at a maximal radial distance from the driving sheave) is constant.
The incorporation of heretofore known proposals to select the pressure of hydraulic fluid by the torque sensor as a function of a plurality variable parameters including a parameter constituting the ratio of the infinitely variable transmission is very expensive and contributes to the bulk of such power trains. The reason is that the reliance upon a second parameter, particularly the ratio of the transmission, is effective or reliable only if the machining tolerances are reduced to a minimum which is the primary cause of increased cost. Moreover, it is necessary to resort to a relatively large (or extremely large) number of additional conduits, bores and/or other passages as well as to additional valves which are or which can be quite complex and expensive and occupy a substantial amount of space. Reference may be had, for example, the the aforementioned published German patent application No. 42 01 692.
The disclosure in the patent application titled: "Power Train", Ser. No. 08/572,538, filed Dec. 14, 1995, inventor: Oswald Friedmann is hereby incorporated herein by this reference; and the disclosure in the patent application titled: "Power Train with Infinitely Variable Ratio Transmission", Ser. No. 08/772,536, filed Dec. 14, 1995, inventors: Oswald Friedmann, Urban Panther and Ivo Agner, is also hereby incorporated herein by this reference.